Granular flows in fluid

Avalanches, landslides, and debris flows are geophysical hazards, which involve rapid
mass movement of granular solids, water, and air. Globally, landslides cause billions of
pounds in damage, and thousands of deaths and injuries each year. Hence, it is important to
understand the triggering mechanism and the evolution of flow. The momentum transfer
between the discrete and continuous phases significantly affects the dynamics of the flow
as a whole. Although certain macroscopic models are able to capture simple mechanical
behaviours, the complex physical mechanisms occurring at the grain scale, such as
hydrodynamic instabilities, formation of clusters, collapse, and transport, have largely
been ignored. In particular, when the solid phase reaches a high volume fraction, the strong
heterogeneity arising from the contact forces between the grains, and the hydrodynamic
forces, are difficult to integrate into the homogenization process involving global
averages. In order to describe the mechanism of immersed granular flows, it is important
to consider both the dynamics of the solid phase and the role of the ambient fluid. The
dynamics of the solid phase alone are insufficient to describe the mechanism of granular
flow in a fluid; it is important to consider the effect of hydrodynamic forces that reduce the
weight of the solids inducing a transition from dense-compacted to dense-suspended flows,
and the drag interactions which counteract the movement of the solids. Transient regimes
characterized by change in solid fraction, dilation at the onset of flow and development of
excess pore pressure, result in altering the balance between the stress carried by the fluid
and that carried by the grains, thereby changing the overall behaviour of the flow. In the
present study, 2D Lattice-Boltzmann and Discrete Element Method is adopted to capture
the fluid-soil interactions in underwater avalanches.